Variable resistor



June 18, 1940.

S. RUBEN VARIABLE REsIsT'oR I Filed July 17, 1936 INVENTOR SAMUEL RUBENBY m ATTORNEY Patented June 18, 1940 UNITED STATES PATENT OFFICEApplication July 17,

4 Claims.

Another object is the provision of a variable resistor device of thewire-wound type in which a sheath of metal is directly cast upon theresistor element to aord a rapid heat transference.

A further object is the provision of a variable resistor device of thewire-wound type in which the wire is integrally coated with a thin,comparatively flexible inorganic insulating coating thermally bonded tothe wire and which has a supporting housing of metal cast directly uponand around the insulated wire.

Another object is to provide a variable resistance element having a highrate of heat dissipation and a negligible temperature gradient betweenthe active resistor and a surrounding intimately-contacting metal body.

Other objects will be apparent from the disclosure and the drawing inwhich- Fig. 1 represents a plan view of a variable resistor constructedaccording to the invention;

Fig. 2 is a cross-section taken on the line 2 2 0f Fig. 1;

Figs. 3 and 4 show detailed views at the ends of the resistor coil; and

Figs. 5 and 6 are magnied views of portions of the resistance winding.

This application is a continuation in part of my co-pending applicationsSer. No. 67,599, led March 7, 1936, and Ser. No. 71,365, filed March 28,1936.

Broadly the present invention may be said to comprise a variableresistor of the wire-wound type in which the resistance wire has aninsulation of inorganic material integral with its surface, part of saidinsulation being cut away to permit contact with the wire by a movablecontact element, the insulated portion of the wire being embedded in ametal support which is directly cast upon the insulation, thus providingan immediate heat transference from the resistance element to theexterior.

The inorganic insulating coating is best ap- 1936, Serial No. 91,027

plied to the resistance wire by the electro-deposition method describedin my co-pending application Ser. No. 67,599. The wire passes through asolution in which finely-divided insulating material, such as colloidalkaolinite is suspended in 'an organic silicate such as ethyl silicate.The resistance wire is positively charged and the container in which thesolution is held is negatively charged. As the wire is pulled throughthe s olution a thin coating of kaolin and silicate is elec- 10trolytically deposited upon the wire which is then passed through anoven which bakes and hardens the insulation. A continuous process isused, the wire preferably passing through the solution a number of timesat a temperature of about 160 15 C., being baked after each immersion toinsure a non-porous coating. The repeated passes eiect a self-healingaction inasmuch as any weak or porous areas remaining from the previouspass will be covered up. In the electro-deposition 20 process thepotential between the wire and the metal container in which the solutionis held may be about 6 volts, depending upon 'the speed of wire travelthrough the solution, the concentration of the colloid, the size of thewire, etc.

e following formula and procedure for coating nickel-chromium Wire ofsmall diameter has given excellent results:

500 g. of ethyl silicate are mixed with 100 g. of alcohol and 9 g. ofwater. After standing for 24 30 hours g. of water and 130 g. ofcolloidal kaolin (kaolinite) are added and the entire mixtureballmilled. Should the solution tend to gel, an organic inhibitor in anamount up to 10% of the weight of the kaolin may be added.

Instead of the mixture described above, I may coat the wire with asolution as described in my co-pending application Serial No. 71,365, asfollows:

200 g. of Garnett shellac is dissolved and com- 40 bined with 1,000 c.c.of a 10% ammonia solution which is stirred and heated to C. tocompletely dissolve the shellac and eliminate any excess of freeammonia. This solution is then diluted with 3500 c.c. of` water and 1125g. of 45 kaolinite added. The mixture is then ball-milled for 10 hours.

The coated wire is flexible, resistant to alkalies and acids and can beheated to high temperatures without leakage. If desirable,'a top coat 50of ethyl silicate may be applied to the coated wire in order to increaseits abrasion resistance.

In order that a flexible coating may be obtained it is practicallynecessary to limit the thickness of the coating to not more than 3 mils.55

` heat dissipation.

For most purposes a coating of from 1 to 2 mils is amply suiiicient. Thewire so coated, considering its inorganic nature, is remarkably flexibleand may be wound upon small or irregularly-shaped metal mandrels orsupports without cracking. A

Other suitable finely-divided suspendable inorganic insulating materialsmay be substituted for the kaolinite, for instance titanium oxide. Themetal housing cast around the insulated wire should have a melting pointlower than that of the coating used. In general application, I prefer azinc alloy casting containing aluminum and copper.

The resistance wire used in the device of this invention can be ofunusually small diameter because of the immediate and intimate contactbetween the coated wire and the metal cast or compressed about it. Dueto the absence of heatinsulating substances such as enamel, asbestos orother materials commonly used in the prior art, the variable resistor ofthis invention is capable of much higher power dissipation than thedevices previously used.

In order to more particularly describe the invention reference is madeto the drawing in which the insulated wire 2 comprising a metal corehaving a thin integral kaolinite deposit thermally bonded thereto, ashereinabove described, is wound upon arc-shaped aluminum form i. Theends 3 of the resistance wire are brought out and held in contact withbronze terminals 4 by folded mica sheets 5 which insulate terminals Ifrom the extensions of the aluminum form I in which they are held. Thebronze terminals l are bent to provide a contact washer at la forsecuring to binding posts Ilia. A flat basemetal alloy frame I is casearound and in direct contact with the coated wire winding 2, said framehaving integral flanges 6a to aid in the Extending through and at rightangles to thn base of the frame 6 is metal control shaft l held withinbushings I2 and I6 and carrying a metal control arm 8, insulatedtherefrom by insulating bushing I2 and insulating washer I3. Contact 9,carried by arm 8,

makes direct surface contact with the top of resistance wire winding 2,the coating 30a of which has been removed over the area of contact so asto expose metal core 30 as seen more particularly in Figures 5 and 6.Flat spring I0 serves to hold contact 9 under pressure against the wiresurface. Nut l5 secures bushing I6 in the frame S. Sleeve 32, secured tothe shaft by pin 3l, holds insulating bushing I2 onto shaft 'I and nutI4 holds insulating washer I3 in place. The shaft 'I carries a knob II,preferably of insulating material. Arm 8 at its end opposite the contactis soldered or welded to a pigtail conducting wire II, the other end ofwhich is secured by center binding post ISa comprising the threaded endof screw I9 which is insulated by insulating washer I8 and insulatingsleeve 2U from the frame 6. Screw I9 carries nuts 2I for securing theterminals of the circuit.

The resistant wire winding connects with terminal posts Isa which aredesigned for having connected therewith terminals for the circuit to becontrolled. 0n posts Ila are mounted bronze washers la. and nuts 2 Ia.

I claim:

l. A resistance element for a variable resistor adapted to operate athigh temperatures com- Saugen prislng a coil of resistance wire havingover substantially its entire surface and bonded thereto by a silicatebinder, a thin, hard, uniform, nonsoftening adherent coating comprisinga plurality of layers of extremely finely divided high heat resistantparticles of inorganic insulating materials of the group comprising highheat resistant silicates and oxides. said resistance element coil beingclosely wound and the turns thereof being unseparated other than by theinsulating coating on the wire, a portion of the insulation being cutaway so as to leave a bare section across said coil whereon a movableconductor may make electrical contact with said coil, and a cast metalhousing in direct contact with said coil and covering a substantial partthereof.

2. A resistancel element for a variable resistor adapted to operate athigh temperatures comprising a coil of resistance wire having oversubstantially its entire surface and bonded thereto, a thin, hard,uniform, non-softening adherent coating comprising a plurality of layersof extremely finely divided high heat resistant particles of inorganicinsulating materials of the group comprising high heat resistantsilicates and oxides, said resistance element coil being closely woundand the turns thereof being unseparated `other than by the insulatingcoating on the wire, a portion oi' the insulation being cut away so asto leave a bare section across said coil whereon a movable conductor maymake electrical contact with said coil, and a cast metal housing indirect contact with said coil and covering a substantial part thereof.

3. A resistance element for a variable resistor adapted to operate athigh temperatures comprising a coil of resistance wire having oversubstantially its entire surface and bonded thereto, a thin, hard,uniform, non-softening .adherent coating comprising a plurality oflayers of ex tremely finely divided high heat resistant particles ofinorganic insulating materials of the group comprising high heatresistant silicates and oxides, said resistance element coil beingclosely wound and the turns thereof being unseparated other than by theinsulating coating on the wire, a portion of the insulation being cutaway so as to leave a bare section across said coil whereon a movableconductor may make electrical contact with said coil, and an aluminummetal housing cast directly in contact with said coil and covering asubstantial portion of the coated section thereof.

4. A resistance element for a variable resistor adapted to operate athigh temperatures comprising a coil of resistance wire having oversubstantially its entire surface and bonded thereto, a thin, hard,uniform, non-softening adherent coating comprising a layer of extremelyfinely divided high heat resistant particles of inorganic insulatingmaterials of the group comprising high heat resistant silicates andoxides, said resistance element coil being closely wound and the turnsthereof being unseparated other than by the insulating coating on thewire, a portion of the insulation being cut away so as to leave a baresection across said coil whereon a movable conductor may make electricalcontact with said coil, and a cast metal housing in direct contact withthe insulated portion of said coil and covering a substantial partthereof.

SAMUEL RUBEN.

